1. Filed of the Invention
The present invention relates to a method of producing iron nuggets, and more particularly, an improved method which can produce metallic iron nuggets which have a high Fe purity and are excellent in transporting and handling due to a large grain diameter with a high yield and good productivity, when they are produced by reducing and melting raw material containing iron oxide such as iron ore and carbonaceous reducing agent such as coke.
2. Description of the Related Art
In connection with the methods for producing metallic iron nuggets by heating and reducing raw material containing an iron oxide source such as iron ore and carbonaceous reducing agent such as coke and coal powder, and subsequently heating and melting it, many proposals have already been made as can be seen from, for example, Japanese Patent Laid-open Publications Nos. Hei 9-256017, Hei 11-335712 and etc. In these publications, it was found that the improvement of productivity of metallic iron nuggets can be accomplished by heating reduced iron produced by reduction and lowering a melting point of the reduced iron by carburizing the reduced iron, because its melting is promoted.
However, while these publications disclose that metallic iron nuggets with high Fe content can be obtained by making metallic iron nuggets while separating slag collaterally produced in the reduction process, when the metallic iron produced by reduction is molten, it can not be said that reviews as to how to produce the metallic iron, with a high yield, as metallic iron nuggets having a grain size distribution which renders it easy to transport and handle them has been made so sufficiently. Furthermore, depending on the quality of raw material and mixture, sometimes it may not be possible to obtain metallic iron nuggets having an appropriate grain diameter range as a product with a high yield, if the metallic iron nuggets contains slag due to insufficient separation of the metallic iron from the collaterally produced slag and slack proceeding of coalescing or becomes so fine granulated product that its separation becomes difficult in the melting process after reduction.
Additionally, in Japanese Patent Laid-open Publication No. Hei 9-310011, there is disclosed a technique for stabilizing the quality and the productivity of reduced iron obtained by restraining the volatile matter content contained in the carbonaceous material to prevent the bursting of pellets, when producing reduced iron pellets by heating and reducing the pellets containing carbonaceous material. However, the invention disclosed in the publication is to produce reduced iron by heating and reducing the iron oxide in the raw material pellets without melting the iron oxide and thus, although metallization rate itself is increased, the Fe purity itself of obtained reduced iron is considerably low because the obtained reduced iron contains a large quantity of slag component. Also, because this invention relates to a method of producing reduced iron which is not in the molten state, it is different from the technique for obtaining metallic iron nuggets of high Fe purity excluding slag component, and there is no doubt that the object for producing metallic iron nuggets of a target grain size distribution with a high yield does not arise from this invention.
Besides the above technique, numerous methods for producing reduced iron by heating and reducing raw material containing an iron oxide source and carbonaceous reducing agent or for producing metallic iron nuggets by re-melting the metallic iron after the heating and reducing process have been proposed. However, it is difficult to say that the hitherto known techniques for producing metallic iron nuggets are established as concrete techniques for producing metallic iron nuggets having an appropriate grain diameter range with a high yield by taking quality, handling and the like as raw material for iron manufacture, steel-making and alloy steel-making into collective consideration.
And, while it is desirable to use processed carbonaceous material such as coke and oil coke or good quality carbonaceous material such as smokeless coal and bituminous coal having a high fixed carbon content ratio, as carbonaceous reducing agent used for the raw material in producing metallic iron nuggets as explained in the above, recently, it becomes frequent to inevitably use low quality coal, such as brown coal according to the depletion of good quality coal, and if low quality coal is used as carbonaceous reducing agent, the quality or the yield of metallic iron nuggets which is finally obtained will be reduced and more particularly, various unexpected problems will be arisen when it is intended to obtain metallic iron nuggets of an appropriate grain size distribution.
The present invention is conceived considering the above situation, and it is an object of the present invention to establish a method of having a high Fe purity and producing metallic iron nuggets having an appropriate grain size distribution with a high yield and good productivity even when using low quality carbonaceous material as well as when using good quality carbonaceous material as carbonaceous reducing agent.
The producing method of solving the above problems according to the present invention comprises steps of heating raw material containing carbonaceous reducing agent and iron oxide-containing material in a reducing/melting furnace, reducing the iron oxide in the raw material, and then heating the produced metallic iron and simultaneously making it coalesce while separating the metallic iron from slag component, and is characterized in that the method uses carbonaceous reducing agent having a high fixed carbon content ratio.
As the above carbonaceous reducing agent that may be used in embodying the present invention, the fixed carbon content ratio contained in the reducing agent is preferably at least 73% (which means mass percent and is used same meaning herein below), and more preferably, at least 74.5%, the volatile matter content contained in said raw material is restrained to be preferably not more than 3.9%, and more preferably, not more than 3.2%, and the mixing content of carbonaceous reducing agent is restrained preferably to be not more than 45%, and more preferably, not more than 44% in relation to iron oxide component contained in iron oxide-containing material of the raw material. Furthermore, it is desirable if the temperature at the time when the metallic iron produced by reduction is molten in said reducing/melting furnace is controlled preferably to be at least 1400xc2x0 C. or 1460xc2x0 C., because it becomes possible to obtain metallic iron nuggets having a relatively large grain diameter, with more high productivity.
As the above carbonaceous reducing agent in the range of preferable fixed carbon content, besides using only carbonaceous material with high fixed carbon content itself, it is desirable if good quality carbonaceous material with a high fixed carbon content and low quality carbonaceous material with a low fixed carbon content are jointly used and if it is possible to adjust the mixing ratio thereof to secure a predetermined fixed carbon content, because even the low quality carbonaceous material can be used without any trouble. Furthermore, it is desirable if a powdery carbonaceous material is charged adjacent to the raw material before the metallic iron produced by reduction within the reducing/melting furnace, because the powdery carbonaceous material can maintain the atmosphere adjacent to the raw material in a high reductive condition at the terminal stage of reduction, so that re-oxidation of the produced metallic iron can be certainly prevented, and at the same time, the carbonaceous material can serve as a carburization source to the metallic iron, so that the melting and the coalecing of metallic iron can be effectively performed at more lower temperature, thereby contributing to restrain thermal deterioration of the reducing/melting furnace, to reduce heat energy for operation and to improve the productivity of iron nuggets.
And, while the grain diameter of metallic iron nuggets obtained according to the present invention is not particularly limited to a certain range, the preferable range of the grain diameter is 2-50 mm and more preferable range is 3-40 mm, when collectively considering the separation efficiency from the slag collaterally produced at the time of manufacturing or the easiness in transporting and handling as a product and the like. Herein, the term, xe2x80x9cmetallic iron nuggetsxe2x80x9d does not essentially mean those having a true spherical shape and generally refers to all of the products having an elliptical shape, an egg shape, or those somewhat flattened from these shapes, and the above xe2x80x9cpreferred grain diameterxe2x80x9d means a diameter if the metallic iron nuggets takes a true spherical shape, the mean value of long and short diameters if the metallic iron nuggets takes an elliptical or an egg shape, and the value obtained by dividing total sum of long diameter, short diameter and maximum thickness by 3 if the metallic iron nuggets takes somewhat flattened shape. According to the present invention, it is possible to obtain metallic iron nuggets having a grain diameter in the range of 3-40 mm with a yield of at least 80%, and even at least 90%.
And, the xe2x80x9cfixed carbon in the carbonaceous reducing agentxe2x80x9d means carbon component according to the definition and measuring method defined in JIS (Japanese Industrial Standards) M 8812, and the xe2x80x9cvolatile matterxe2x80x9d is also defined in the same standard and contains volatile organic matter of low molecular weight hydrocarbon and the like such as CH4, and absorbed H2, CO, CO2, moisture and the like, and the volatile matter further includes Zn and Pb if ore or some kinds of dust are used as iron oxide-containing material.
As a preferred aspect in embodying the present invention, it is also recommended to reuse carbonaceous reducing agent contained in said raw material which is discharged out of said reducing/melting furnace together with said metallic iron nuggets or a part or all of powdery carbonaceous material as carbonaceous reducing agent in said raw material.
As described in the above, according to the present invention, when producing metallic iron nuggets by reducing raw material containing iron oxide-containing material such as iron ore, iron oxide, or partially reduced products thereof (hereinafter, sometimes referred to as iron oxide source or iron ore and the like) and carbonaceous reducing agent such as coke or coal (hereinafter, sometimes referred to as carbonaceous material), and heating the produced metallic iron to be molten and simultaneously making the produced metallic iron coalesce while separating it from collaterally produced slag component, reducing agent having a high fixed carbon content is selected and used as said carbonaceous reducing agent serving as reducing agent, in particular for the iron oxide source in the raw material, and furthermore, the volatile matter content in the raw material, or the mixing content of the carbonaceous reducing agent to the iron oxide component is properly adjusted, whereby the melting and coalescing of metallic iron is promoted at the time of reducing and melting, and metallic iron nuggets with an appropriate grain size distribution can be obtained with good yield.
As raw material used herein, there is no particular limitation in type if it contains the above iron oxide source and carbonaceous material, and powdered homogeneous mixture, or the raw material may be used in compacted form such as pellets, briquettes or small masses, which are compacted using a binder and preferably have a diameter of about 3-30 mm calculated in terms of the mean grain diameter, in order to obtain metallic iron nuggets having a relatively large grain diameter as intended in the present invention, with more high yield and good productivity.